Thermoplastic elastomers formed from blends of cured rubber and polyolefins are known in the art. The structure of such materials is in the form of a matrix containing a plastic component with discrete domains of a partially or fully cured elastomeric component embedded therein. Olefin-based thermoplastic elastomers, with the structure thus described, have the advantage of being able to undergo plastic flow above the softening point of the polyolefin, and yet behave like a cured elastomer below the softening point. Thus, the materials exhibit thermoplasticity (i.e., flowing at elevated temperature under processing conditions) while also exhibiting rubber-like elasticity (i.e., recovering a substantial amount of deformation when a deforming force is removed).
Dynamic vulcanization is a process whereby the elastomeric portion of the thermoplastic elastomer is cured by heating the blend in the presence of a curative while shearing the blend to form a thermoplastic vulcanizate (TPV). Different curing methods that may be used to partially or fully cure the rubber during dynamic vulcanization include sulfur-, peroxide-, phenolic-, and hydrosilation-based systems.
The extent of cure (i.e., partial or full) of the elastomeric or rubber phase is an important factor in the ultimate properties of the final composite, such that lower oil swell and higher ultimate tensile strength are observed at high states of cure, as taught in U.S. Pat. No. 4,130,535. In other words, a blend containing a fully cured elastomeric phase has improved physical properties as compared to uncured or partially cured blends. Such fully cured vulcanizates are processable as thermoplastics although they are crosslinked to a point where the rubber portions are almost or entirely insoluble in the usual solvents. The processability of a fully cured thermoplastic vulcanizate (TPV) is in direct contrast to thermoset compositions, which retain dimensional integrity at service temperatures of 200° C. or above.
Polar materials, e.g., substrates, are typically either inorganic materials, such as metals, or polymers, such as engineering resins. Commercial olefinic thermoplastic elastomers are generally not easily bonded to polar materials when formed as a multilayer composite in, for example, co-extrusion processes. Usually, the bond is so weak that it delaminates spontaneously or will lose adhesion over a period of time, especially in a moist atmosphere. Various attempts to facilitate adhesion of thermoplastic vulcanizates to polar materials have been attempted in the prior art.
U.S. Pat. No. 4,957,968 relates to an adhesive thermoplastic elastomer composition that includes at least one polyolefin modified by a chemically reactive functional group such as a carboxylic acid or its derivatives including anhydride, acid chloride, isocyanate, oxazoline, epoxide, amine and hydroxide, at least one other polymer prepared from one or more of the following: ethylene, propylene, butylene, isobutylene, octene-1, 4 methyl pentene-1, hexene-1 or mixtures; and at least one partially cured olefinic elastomer, including elastomeric ethylene-propylene copolymers, elastomeric ethylene-propylene terpolymers, polyisoprene, polyisobutylene, ethylene-propylene copolymer rubber, polybutadiene, natural rubber, elastomeric polyesters, polyurethane elastomers, polyamide elastomers and mixtures.
U.S. Pat. No. 5,574,105, for example, describes a thermoplastic elastomer comprising a triblend of an engineering resin, a dynamically vulcanized alloy (DVA) of a thermoplastic olefin polymer and an elastomeric copolymer, and a compatibilizer. The compatibilizer is prepared by melt mixing nylon-6 with polypropylene grafted with 0.1 to 2.0% by weight maleic anhydride. The patent teaches that, without the compatibilizer, blends of engineering resins and DVA have poor mechanical and elongation properties due to the lack of interfacial adhesion between the components, and that, under stress, the weak interfaces may fail and the components delaminate.
U.S. Pat. No. 5,695,838 describes a multilayer laminate body with at least three layers: polyolefin layer, adhesion layer, and polyamide layer. The adhesion layer contains a blend of propylene grafted with an unsaturated carboxylic acid or an anhydride of a dicarboxylic acid and of an ethylene/alpha-olefin copolymer grafted with an unsaturated carboxylic acid.
U.S. Pat. No. 5,843,577 describes the addition of a reaction product of a maleated polypropylene with a polyamide to a dynamically vulcanized thermoplastic elastomer to improve the adhesion of the TPV to an engineering resin such as nylon. The patent further teaches that maleated polypropylene containing about 1.5 wt % of maleic anhydride grafted thereon added to a blend of polypropylene and fully cured EPDM and a common additive with a Durometer Shore A hardness of 45 (Santoprene® 111-45, commercially available from Advanced Elastomer Systems, USA) alone at high level (15 wt %) showed undesirable non-cohesive adhesion performance to polyamide.
U.S. Pat. No. 5,962,146 describes EPDM or EPM rubber coated, without the use of adhesives, with a film containing polyamide and an olefinic material grafted with unsaturated carboxylic acid or an unsaturated dicarboxylic acid or its anhydride.
U.S. Pat. No. 6,300,418 describes a composition containing a polyolefin resin, a rubber such as EPDM or SEBS, and an adhesion promoter in the form of a reaction product of a functionalized rubber and a polyamide. The composition may be at least partially crosslinked.
The prior art addition of polar materials such as engineering resins to the thermoplastic elastomeric blend may not be desirable, however. Physical properties of polar/non-polar blends may be compromised since nylon, for example, is vulnerable to moisture, and non-moisture sensitive TPVs modified with nylon may be damaged from moisture-induced degradation during processing. In addition, adding polyamide to the thermoplastic vulcanizate may adversely affect the morphology and elastic properties of the overall composition, such as increasing compression set. Thus, there remains a need for a thermoplastic elastomeric blend that may be adhered or bonded to a polar substrate without the need for separate adhesion layers, mechanical interlocking, or the addition of undesirable components to either the polar substrate or the thermoplastic vulcanizate composition.